The present invention relates to a method for treating wastewater containing organic matter and nitrogen, and more particularly, to a wastewater treatment method utilizing a carrier capable of enhancing the removal efficiency of organic matter and nitrogen.
An active sludge method, a basic wastewater treatment method, has been widely used in the secondary treatment of wastewater after it has undergone a first treatment, or in order to completely treat the original wastewater in aerobic conditions. In a general active sludge method, as the wastewater continuously flows into an aeration tank, microorganisms grow by intaking the organic matter of the wastewater such that the organic matter is decomposed, and the grown microorganisms coagulate to settle in a terminal settling tank. A portion of the precipitant returns to the aeration tank in the form of an active sludge, and the remaining portion is discarded as waste sludge, such that the amount of microorganisms in the aeration tank is maintained at a suitable level and the nitrogen and phosphorus are removed as the organic matter is decomposed.
Such an active sludge method has been acknowledged as an effective wastewater treatment method. However, this method is not suitable for effectively removing the organic matter and nitrogen at the same time from wastewater containing a high-concentration of organic matter and nitrogen, and increases the surplus sludge. In particular, when wastewater contains a high loading ratio of organic matter, bulking occurs in the settling tank, thereby lowering treatment efficiency. In addition, heterotrophic bacteria grow to excess due to the high loading ratio of the organic matter while growth of autotrophic bacteria which inherently grow slowly is suppressed, so that active nitrification is not possible.
To solve the above problems, a biofilm process has been developed in which a large number of microorganisms fixed to a carrier are used. According to the biological membrane process, it is necessary to maintain the number of microorganisms in an aeration tank, used for wastewater treatment, to a suitable level. To this end, a microorganism-attachable carrier is essential for suspended-growth or attached-growth of the microorganisms. Carrier for the purpose include porous plastics(presented by Sung-yong Choi, in Journal of Korean Society on Water Quality, Vol. 6, No. 1, page 31, 1990), active carbon fiber (Japanese Patent Application No. Heisei 5-167820), polyvinyl alcohol and active carbon (Japanese Patent Application No. Heisei 5-186723) or the like. The material of the carrier is low in hydrophilic properties, so that it is not easy to attach microorganisms to the carrier. Also, the microorganisms not attached to the carrier is low in growth rate, so that the microorganisms flows out of the aeration tank when the wastewater treatment tank is continuously operated.
Also, in the conventional biofilm process utilizing a fixed type carrier, because a biofilm is formed on the surface of the carrier in excess, the amount of dissolved oxygen (DO) supplied to nitrifying bacteria whose growth rate is relatively slow is not enough, so that it is hard to maintain the amount of the nitrifying bacterial to a suitable level. In addition, if the conventional biofilm membrane process utilizes a single carrier, and there is a limitation in the concentration of nitrogen removed.
Meanwhile, ammonia nitrogen of the wastewater is treated in two stages of nitrification and denitrification by a biological treatment method. That is, during nitrification, the ammonia nitrogen is changed into nitrate nitrogen (NO3xe2x80x94N) by aerobic nitrifying bacteria, and during denitrification, denitrifying bacteria oxidize the organic matter using the nitrate nitrogen as an electron acceptor, instead of oxygen the amount of which is insufficient, and reduce the nitrate nitrogen to nitrogen (N2). However, the activity of the nitrifying bacteria which are sensitive to a temperature decrease lowers during the winter season, thus the nitrogen removal efficiency is also rapidly lowered.
To solve the above problems, it is an object of the present invention to provide a method for stably and effectively treating wastewater containing a high-concentration of organic matter and nitrogen, by which the above-mentioned problems of the conventional biofilm process, such as bulking, biofilm slough-off or decrease in treatment efficiency during the winter season, can be solved.
According to an aspect of the present invention, there is provided a wastewater treatment method comprising the steps of: (a) denitrifying wastewater inflowed into an anoxic tank by reducing the nitrate nitrogen into gaseous nitrogen using denitrifying bacteria being resident in the anoxic tank, and organic matter of the inflowed wastewater; (b) making the carbon-to-nitrogen (C/N) ratio of the wastewater having undergone the step (a) suitable for a subsequent nitrification step after inflowing the wastewater into a first aeration tank by decomposing an excessive amount of organic matter using aerobic microorganisms attached to a carrier; (c) changing ammonia nitrogen of the wastewater having undergone the step (b) into nitrate nitrogen after inflowing the wastewater into a second aeration tank by using nitrifying bacteria attached to a carrier; (d) returning part of the wastewater having undergone the step (c) to the anoxic tank, and inflowing the remaining wastewater to a settling tank; and (e) returning part of the sludge discharged from the settling tank to the anoxic tank, discarding the remaining sludge as surplus sludge, and obtaining a supernatant separated from the sludge settled in the settling tank as the treated water.
Preferably, after the organic matter is decomposed by the aerobic microorganisms in the step (b), the resulting wastewater is directly sent to the step (d) if the load of ammonia nitrogen of the wastewater to be treated is small, thus omitting the step (c). Preferably, the wastewater passed through the step (a) is directly sent to the step (c) if the load by the organic matter of the wastewater to be treated is small, omitting the step (b).
Preferably, the carrier used in the step (b) comprises a foamed polymer, powdered active carbon attached to the foamed polymer, and an adhesive for sticking the powdered active carbon to the foamed polymer.
Preferably, the foamed polymer is polyurethane, polystyrene or polyethylene, having a sponge foam or non-woven fabric structure.
Preferably, the adhesive is a mixture of acrylic resin and styrene/butadiene Latex (S/B Latex) in a weight ratio of 70:30xcx9c90:10.
Preferably, the powdered active carbon and the adhesive exist in a weight ratio of 40:60xcx9c50:50.
According to another aspect of the present invention, there is provided a method for preparing the carrier, comprising the steps of: (a) forming a slurry by mixing an adhesive solution and a powdered active carbon in a weight ratio of 90:10xcx9c70:30; (b) coating the slurry on a foamed polymer, and evaporating solvent from the adhesive solution by heating the resulting product; and (c) molding the resulting product of the step (b).
Preferably, the heating of the step (b) is performed at 70xcx9c95xc2x0 C. for 2xcx9c3 hours.
Preferably, the carrier used in the step (c) is a polyvinyl alcohol foam carrier or a cellulose fiber carrier.
Preferably, the cellulose carrier is prepared by the steps of: (a) molding a cellulose fiber into a planar shape; (b) infiltrating the planar cellulose fiber with a foaming composition containing polyvinyl alcohol, a cross- linking agent and a foaming agent; (c) foaming the resulting product of the step (b) by dehydrating; (d) infiltrating the resulting product of the step (c) with a reinforcing composition containing polyvinyl alcohol and a cross-linking agent; and (e) drying the resulting product of the step (d) taken out of the composition.
Preferably, the foaming composition comprises 0.5xcx9c5 wt % of polyvinyl alcohol, 0.2xcx9c5 wt % of cross-linking agent, 0.1xcx9c1 wt % of foaming agent, and solvent as the remainder.
Preferably, the reinforcing composition comprises 1xcx9c5 wt % of polyvinyl alcohol, 0.1xcx9c5 wt % of cross-linking agent, and solvent as the remainder.
Preferably, the cross-linking agent is melamine urea resin or polyamide polyamine epichlorhydrine (PPE).
Preferably, dehydration of the step (c) is performed such that the water content of the planar cellulose fiber before foaming is maintained at 20xcx9c50% based on the weight of the planar cellulose fiber.
Preferably, foaming in the step (c) is performed at 120xcx9c150xc2x0 C. such that the volume of pores per unit gram of the foamed cellulose carrier on the dry basis is in the range of 0.3xcx9c4.5 cm3.